Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Electrodepositable primer coating compositions, particularly those used in the automotive industry, typically are corrosion-resistant epoxy-based compositions crosslinked with polyisocyanates. In some applications, a primer-surfacer is spray-applied directly to the cured electrodeposited coating prior to application of one or more top coats. The primer-surfacer can provide a variety of properties to the coating system, including protection of the electrodeposited coating from photodegradation. Alternatively, one or more top coats can be applied directly to the cured electrodeposited coating and in such instances, the top coat(s) are formulated such that the top coat provides sufficient protection to prevent photodegradation of the electrodeposited primer coating.
Various approaches to lowering the photosensitivity of electrodepositable coatings include, inter alia, the use of blocked aliphatic polyisocyanate cross-linking agents. However, excessive cost makes the use of such crosslinking agents undesirable. Accordingly, there remains a need in the coatings industry for a cost effective alternative to conventional aliphatic polyisocyanates, which retards photodegradation of electrodepositable coatings.